Earlier research has demonstrated that treatment with hyperbaric oxygen (HBO 2 ) can elicit an antinociceptive response in models of acute pain. We have demonstrated that this antinociceptive effect is centrally-mediated and is dependent on opioid receptors. The purpose of the present study was to examine the role of endogenous opioid peptides and opioid receptors specifically in the spinal cord in the acute antinociceptive effect of HBO 2 in mice. Male NIH Swiss mice were exposed to HBO 2 (100% oxygen at 3.5atm absolute) for 11min and their antinociceptive responsiveness was determined using the glacial acetic acid-induced abdominal constriction test. HBO 2 -induced antinociception was sensitive to antagonism by intrathecal (i.t.) pretreatment with the κ- and μ-selective opioid antagonists norbinaltorphimine and β-funaltrexamine, respectively, but not the δ-selective antagonist naltrindole. The antinociceptive effect of HBO 2 was also significantly attenuated by i.t. pretreatment with a rabbit antiserum against rat dynorphin 1–13 but not antisera against β-endorphin or methionine–enkephalin. Based on these experimental findings, the acute antinociceptive effect of HBO 2 appears to involve neuronal release of dynorphin and activation of κ- and μ-opioid receptors in the spinal cord.